1. Field of the Invention
The present invention relates to a method for manufacturing laminated dielectrics used as materials for electronic components such as piezoelectric actuators and the like and, more particularly, to a method for manufacturing laminated dielectrics consisting of a laminated body of oxide dielectric layers such as PZT etc. and inner electrode layers containing a base metal as a main constituent.
2. Description of the Related Art
Laminated dielectrics utilized in electronic components such as piezoelectric actuators, and the like, are formed generally by alternately laminating green sheets of oxide dielectric material such as PZT (lead zirconate titanate) and metal layers to be used as inner electrodes, and by integrally sintering the laminated body as one unit. Conventionally, noble metals such as Pd, Pt, Pdxe2x80x94Ag, etc. are widely used as materials for the inner electrodes. However, noble metals are very expensive and increase the manufacturing cost.
Thus, in recent years, attention has been increasingly paid to base metals (CU, N1, etc.) as the material for the inner electrodes in laminated dielectrics. Although, besides being inexpensive, base metals have other advantages such as good migration resistance, there are also problems associated with the fact that base metals tend to be easily oxidized in an oxidizing atmosphere at high temperatures. Therefore, in sintering a laminated body that employs a base metal as inner electrodes, it is necessary to bake the laminated body in a neutral or reducing atmosphere in order to avoid deterioration of the function of electrodes.
As the neutral or reducing atmosphere, a mixed gas of N2, H2, CO, CO2, H2O, etc. has been typically used. In order to facilitate the adjustment of the oxygen concentration in the atmosphere, Japanese Patent Publication (Kokai) No. 05-335177, for example, discloses a method in which a mixed gas having CO2 or CO gas of high purity as a main component and containing H2 and O2 gases optionally adjustable in concentration, is employed. With this method, the concentration of oxygen can be quickly adjusted to a target value by varying the supply of H2 and O2 gases.
When, however, compounds containing PbO are used in the composition as in oxide dielectrics such as PET, if H2 is contained in the atmosphere gas during sintering, reduction of PbO takes place at temperatures of 300xc2x0 C. and higher, and the resulting Pb may diffuse, leading to the problem of fluctuations in the composition. Further, when the oxide dielectric contains alkaline earth metals in the composition, if H2O is contained in the atmosphere, the alkaline earth metals may react with H2O and lead to expansion of the laminated body (room temperature xcx9c400xc2x0 C.), allowing only small-sized laminated dielectrics to be fabricated.
It is an object of the present invention to resolve the above described problems and to provide a method, for manufacturing large-sized laminated dielectrics of high quality, which permits adjustment of the atmosphere during sintering process to be accomplished easily, and which can avoid the problems such as fluctuations of the composition of oxide dielectrics and expansion due to reaction with the atmosphere gas.
According to a first aspect of the invention, a method for manufacturing laminated dielectrics is provided wherein a laminated body consisting of oxide dielectric layers and inner electrode layers containing base metals as the main component is sintered in a reducing atmosphere characterized in that the above mentioned sintering step is performed in a mixed gas having CO2 gas as the main constituent and optional amount of CO gas and O2 gas, and that the oxygen partial pressure in the atmosphere is adjusted within a predetermined range by adjusting the amount of the above mentioned CO gas and O2 gas.
According to the present invention, sintering of laminated dielectrics is performed in a mixed gas having CO2 gas as a main constituent, and further containing CO gas and O2 gas of arbitrarily adjustable concentrations in order to control the oxygen partial pressure in the atmosphere gas at the target oxygen partial pressure. Thus, if the atmosphere in the oven deviates from this target value, amount of supply of CO gas or O2 gas can be increased or decreased so as to quickly adjust the oxygen partial pressure to within a predetermined range. Problems with the conventional method in which H2 gas and H2O are contained in the atmosphere, such as the fluctuation of the composition of the dielectrics or the expansion of the dielectrics, do not arise with the method of the present invention.
The atmosphere in the oven is maintained in an equilibrium of CO2 gas, CO gas and O2 gas determined by the equation CO2+CO⇄2CO+(1/2)O2. By adjusting the amount of CO gas and O2 gas utilizing this equilibrium system, the oxygen partial pressure in the atmosphere can be easily and stably controlled, and the variation and fluctuation of the atmosphere can be suppressed. Therefore, the atmosphere in the oven can be easily adjusted during a sintering process, and large-sized laminated dielectrics having good appearance and high quality can be obtained.
According to a second aspect of the invention, during the above mentioned sintering step, it is desirable to maintain the ratio of gas components in the mixed gas supplied to the oven preferably in the range of CO2:CO:O2=5000:500xcx9c0:20xcx9c0, and to maintain the oxygen partial pressure in the atmosphere gas within a proper range so that melting of the base metal electrodes and diffusion of the electrode into the composition can be suppressed.
More specifically, according to a third aspect of the invention, during the above mentioned sintering step, it is desirable to adjust the oxygen partial pressure of the atmosphere gas in the range, depending upon the sintering temperature, such that the reduction of the oxide composing the above mentioned oxide dielectric layers does not occur and the electrical conductivity of the base metal composing the above mentioned inner electrode layers is not impaired. Since the range of oxygen partial pressure in which oxidative diffusion of the base metal electrodes can be suppressed and fluctuation of the composition of the dielectric component can be avoided varies with the sintering temperature, the atmosphere should be adjusted to achieve the optimum oxygen partial pressure depending upon the sintering temperature in order to obtain the laminated dielectrics of better quality and higher reliability.
According to a fourth aspect of the invention, preferably, a reduction processing step in which the above mentioned laminated body is subjected to heat treatment in a reducing atmosphere is provided for performing reduction processing on the inner electrode layers prior to the above mentioned sintering step. During this reduction processing step, the oxygen partial pressure of the atmosphere gas (oxygen partial pressure outside the oven) is preferably maintained in the range of 1xc3x9710xe2x88x9214xcx9c1xc3x9710xe2x88x9225 atm so that metallizing process can be accomplished while suppressing the reduction of the oxide dielectrics and avoiding melting of the base metal electrode and diffusion of the electrode into the composition.
According to a fifth aspect of the invention, processings such as sintering etc. can be advantageously performed by disposing aerating boards, formed of materials having no reactivity with the above mentioned laminated body, in generally symmetrical positions on the upper and lower sides, or in a position either on the upper or lower side of the laminated body, so as to be in contact therewith. The above mentioned aerating board can be formed of materials having substantially no reactivity with the laminated body and having aerating pores, for example, oxides of alkaline earth metals such as MgO.
According to a sixth aspect of the invention, a spacer layer consisting of a honeycomb body, a porous structure, a mesh plate or a mesh body, composed of a ceramic or a metal identical to any one of the elements contained in the inner electrode layers of the above mentioned laminated body, may be disposed in generally symmetrical positions on the upper and lower sides, or in a position either on the upper or lower side of the laminated body. When the above mentioned spacer layer having good gas permeability is disposed on the upper and/or lower sides of the laminated body, the atmosphere on the upper and/or lower sides of the laminated body is advantageously homogenized during the sintering and reduction processings.
According to a seventh aspect of the invention, a weight composed of a ceramic or a metal identical to any one of the elements contained in the inner electrode layers of the above mentioned laminated body, may be disposed on the upper side of the laminated body. The size of the above mentioned weight is preferably greater than that of the laminated body so that, when processing such as sintering is performed in this state, the weight can effectively suppress the deformation in the direction of the thickness of the laminated body.
According to an eighth aspect of the invention, in the above mentioned reduction processing step, all or any one or more of the aerating boards, the spacer layers and the weight as described above in the fifth and seventh aspects of the invention, may be used. By appropriately combining these as required, the atmosphere for processing can be diffused evenly over the whole laminated body, thereby eliminating a fluctuation in the reduction of the inner electrode layers so as to obtain an improved quality of the product.
According to a ninth aspect of the invention, in the above mentioned sintering step, all or any one or more of the aerating boards, the spacer layers and the weight as described above in the fifth and seventh aspects of the invention, may be used. By appropriately combining these as required, the sintering atmosphere can be supplied evenly so that the whole laminated body may be sintered evenly and problems such as delamination or cracks may be eliminated.
According to a tenth aspect of the invention, in the sintering step or the reduction processing step, gas permeability for the atmosphere gas is preferably substantially the same on the upper surface and the lower surface of the laminated body. In this manner, the atmosphere gas can be supplied evenly to the upper and lower surfaces of the laminated body so that, when the reduction processing has been accomplished, the amount of reduction of base metals in the inner electrode layer may be made even throughout the device. Also, problems such as diffusion of base metals into the device after the sintering step, defects in the external appearance of the device, and the like, can be avoided.
According to an eleventh aspect of the invention, the present invention can be advantageously applied to the case where total volume of the above mentioned laminated body is as large as 8 mm2. When sintering is performed according to the method of the present invention, a difference in the distribution of the atmosphere gas supplied to the above mentioned laminated body can be made small enough to permit even sintering so that large-sized laminated dielectrics of high quality may be realized.
According to a twelfth aspect of the invention, the present invention can be advantageously applied to the case where the above mentioned laminated body has a thickness of 2 mm or more and an area of 4 mm2 or larger in the direction parallel to the inner electrode. When sintering is performed according to the method of the present invention, difference in the distribution of the atmosphere gas supplied to the above mentioned laminated body can be made small enough to permit even sintering so that large-sized laminated dielectrics of high quality may be realized.
According to a thirteenth aspect of the invention, the present invention can be most effectively applied to the case where the above mentioned oxide dielectric layer contains lead, in the composition, such as a PZT type material. In this case, deterioration of the quality of the product can be avoided by stably controlling the oxygen partial pressure in the atmosphere gas,and thereby suppressing the reduction of the lead oxides.
According to a fourteenth aspect of the invention, layers containing copper as the main component element are advantageously used as the inner electrode layers of the above mentioned laminated body, and are desirable since they are inexpensive and have good migration resistance.